Light-emitting diodes (LEDs) are used in a wide variety of applications, including in high-power lighting and illumination. LEDs present many advantages over conventional incandescent sources for lighting and illumination, including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. In particular, there is ongoing research to develop blue light-emitting diodes that can be mixed with red and green LEDs or can be used to excite a phosphor material to form white light for general lighting applications. Low-power LEDs, typically operating at a few milliamps, have been shown to have high luminous efficacy of greater than 100 lm/W. However, practical general lighting requires high-power LEDs capable of one watt or more. Typical operating currents for such high-power LEDs are several hundred mA to more than an ampere. A recurring problem with LEDs is the gradual decrease of the power efficiency as the injection current increases, an effect known as “efficiency droop.”
In particular, efficiency droop has been shown to be a unique characteristic of GaN-based multiple-quantum-well (MQW) LEDs. The origin of the high-power loss mechanism remains under active investigation. Several loss mechanisms have been suggested, including electron leakage out of the MQW active region, low hole injection, Auger recombination, carrier delocalization occurring at high excitation intensities, and junction heating. See M. H. Kim et al., Appl. Phys. Lett. 91, 183507 (2007); M. F. Schubert et al., Appl. Phys. Lett. 91, 231114 (2007); M. F. Schubert et al., Appl. Phys. Lett. 93, 041102 (2008); J. Xie et al., Appl. Phys. Lett. 93, 121107 (2008); I. V. Rozhansky and D. A. Zakheim, Phys. Status Solidi A 204, 227 (2007); I. A. Pope et al., Appl. Phys. Lett. 82, 2755 (2003); Y. C. Shen et al., Appl. Phys. Lett. 91, 141101 (2007); N. F. Gardner et al., Appl. Phys. Lett. 91, 243506 (2007); A. Y. Kim et al., Phys. Status Solidi A 188, 15 (2001); S. F. Chichibu et al., J. Vac. Sci. Technol. B 19, 2177 (2001); and A. A. Efremov et al., Semiconductors 40, 605 (2006). For blue LEDs injected with high current densities, the physical mechanism causing the efficiency droop was found to be the single largest loss mechanism. See M. F. Schubert et al., Appl. Phys. Lett. 91, 231114 (2007).
Therefore, a need remains for the mitigation of the efficiency droop GaN-based and other III-Nitride LEDs that may enable the pervasive market penetration of solid-state-lighting technologies.